Many patients with lung and/or cardiovascular problems may be required to breathe therapeutic gas in order to obtain sufficient dissolved oxygen in their blood stream. So that these patients may be ambulatory, therapeutic gas may be delivered from a portable cylinder. A portable cylinder may, however, provide only limited volume, and therefore periodically needs to be refilled. While it is possible to have these cylinders exchanged or refilled by way of commercial home health care services, some patients have systems within their homes which generate therapeutic gas and fill portable cylinders with the therapeutic gas. Systems such as these have come to be known as transfer-fill or “trans-fill” systems. However, the trans-fill systems of the related art fill the portable cylinders very slowly, and are prohibitively expensive for most patients.